During the midweek event his week, I repeatedly tried to cast spells using the enweb ability. the mana from the omniscience "add" emblem would provide me with the mana needed to do so. I even tried to decidedly forced the add. However no matter how often or under which circumstances I tried, the "enweb" side of the card was always greyed out, i.e. non-usable and I was forced to cast the card for the 0 mana omniscience cost.

if you are wondering why I would do so: I had decidedly drafted cards with enweb together with other cards which would give me an advantage when entering the battlefield or being cast, so the enweb woul provide me with the opportunity to reanact thos abilities. Being blocked out from this option was rather annoying and although I find the omniscience gameplay interesting, I am loathe to enter Through The Omenpaths Omniscience Quick Draft for this very reason.

I have now read on this forum, that in cases where a spell would have an X in its cost, you would face a similar problem, which may be solved by clicking ctrl while casting the spell. I suspect this workaround might also work for the enweb problem but cannot verify.
Also, I think the game should auto-recognize this as a valid option and not force you into using shortcuts.